Objective-C Fundamentals

40 CHAPTER 2 Data types, variables, and constants
Unlike some languages with a true Boolean data type, Objective-C doesn’t restrict
variables of type BOOL to storing only the two values YES and NO. Internally, the framework
defines BOOL as another name for the signed char data type discussed previously.
When Objective-C evaluates a Boolean expression, it assumes any nonzero value
indicates true and a value of zero indicates false. This choice of storage formats can
lead to some interesting quirks. For example, the following code snippet tries to suggest
that a BOOL variable can store values of both true and false at the same time:
BOOL result = 45;
if (result) {
NSLog(@"the value is true");
}
if (result != YES) {
NSLog(@"the value is false");
}
This quirk occurs because the result variable stores a nonzero value (hence indicating
truth) but doesn’t store exactly the same value that YES is defined as (1). In general,
it’s best to compare BOOL values against the value NO. Because only one value can ever
indicate false, you can be assured hard-to-spot errors such as the one demonstrated
here don’t occur in your source code.
This completes our initial coverage of how to declare and store values of built-in
primitive data types. You still have much to learn about them, though, and we continue
to discuss them throughout the rest of the book.
2.3 Displaying and converting values
Using variables to store values is useful, but at some stage, you’ll want to display them
to the user. Usually this will necessitate converting a raw value into a nicer, more formatted
form. For example, a floating-point calculation may result in the value
1.000000000000276, but a user may not be interested in an answer to this level of precision
(if indeed the result was ever that precise given the potential inaccuracies discussed
in section 2.2.2). It may be more suitable to present this value only to two
decimal places, as 1.00.
In the following section, we discuss in detail how you can alter the display of arguments
provided to a call to NSLog by altering what is called the format string. We also
take a brief look at how numeric values can be converted between the various representations
and how this too can affect the results of a calculation.
2.3.1 NSLog and Format Specifiers
The first argument provided in a call to NSLog specifies what is called the format string.
NSLog processes this string and displays it in the Xcode debugger console. In most
cases, the format string contains one or more placeholders that are indicated by a %
character. If placeholders are present, NSLog expects to be passed a matching number
of additional arguments. As NSLog emits its message, it substitutes each placeholder